(1) Field of the Invention
The present invention relates to digital switching systems and more particularly to a pulse toggle monitor circuit for use in such switching systems.
(2) Description of the Prior Art
Pulse toggle monitor circuits detect the duration of toggling of a monitored pulse. Typical pulse toggle monitor circuits (e.g. contact bounce circuits), while being old and well known, are usually analog rather than digital circuits. Pulse monitor circuits, by contrast, detect the presence or absence of a monitored pulse. These circuits do exist in digital form but they require complex logic circuitry and are sometimes unreliable and subject to race conditions. A recent advancement in the state of the art of pulse monitors was disclosed by H. Ballentine in an article entitled "Clock-Activity Detector Uses One DIP", Electronic Design News, Jan. 5, 1980, page 156. However, this circuit is still relatively complex and any detected failure signal is cleared when the monitored clock signal reappears. Some implementations of this circuit may also suffer from potential race problems due to inadequate clear signal pulse widths.
Accordingly it is the object of the present invention to provide a minimum component, highly reliable, digital pulse toggle monitor circuit, free of any potential race problems and resettable by either an external clear signal or appearance of a valid pulse.